Summary of Work: We use the powerful bacteriophage T4 model system to study mechanisms of mutagenesis and DNA repair. (1) Forward mutation in T4 is often measured by counting r mutants. These arise in about five genes. We have now analysed the rV gene. rV mutations arise in the first third of gene t, which was previously defined by a very different mutant phenotype and which encodes the holin that mediates transfer of endolysin to its murein target and thereby controls lysis and the release of progeny phages. (2) We have characterized a new T4 mutation reporter gene, the rI gene, which is useful for mutation studies because it is not involved in DNA metabolism, is small enough to be accommodated by a single DNA sequencing lane, and is very sensitive to missense mutations. Spontaneous mutation is interesting in this gene because of a hot-spot of quasipalindromic mutation. (3) A T4 rnh (RNase H) mutant was reported to be a mutator. We have now shown that a rnh null mutations have few or no discernible effects on mutation rates. In addition, we observed a bizarre epistatic interaction between rnh and rII mutations. (4) Heat can promote G:C -> T:A mutagenesis in extracellular T4 particles. The rate of this reaction in the laboratory has gradually fallen in the past 20 years. We now believe that the mutation was promoted by trace metal-ion contaminants in distilled water or buffer salts, and have terminated the project. - Bacteriophages, DNA Damage, DNA Replication, Escherichia Coli, Mutagenesis, UV Rays